Pr. Levstein et H. Vanwilligen, PHOTOCHEMISTRY IN HETEROGENEOUS MEDIA - A FOURIER TRANSFORM-EPR STUDY, Colloids and surfaces. A, Physicochemical and engineering aspects, 72, 1993, pp. 43-54
Fourier transform-electron paramagnetic resonance (FT-EPR) is a novel
spectroscopic technique that is ideally suited for the study of transi
ent free radicals generated in photochemical reactions. The salient fe
atures of the application of FT-EPR in this field of research are the
following. (1) The high spectral resolution of the technique makes it
possible to identify free radicals. (2) The time evolution of spectra
can be monitored over a time regime extending from nanoseconds to mill
iseconds. Hence, spectral data can give rate constants of radical form
ation and decay. (3) Signal intensities can reflect chemically induced
electron polarization effects that provide information on the mechani
sm of radical formation. (4) Measurement of spin-relaxation rates give
s an insight into molecular motion. In this paper the application of t
he technique in a time-resolved study of photoinduced electron transfe
r in micellar solutions and in the pores of silica gel is presented. M
easurements on micellar solutions concern electron transfer from photo
excited tetrasulfonated zinc tetraphenylporphyrin to duroquinone. The
study of silica gel systems focused on electron transfer from zinc tet
raphenylporphyrin to benzoquinone. The results of the investigation sh
ow that the technique can provide valuable insights into photochemical
reactions in heterogeneous media. The time evolution of the spectra p
rovides information on the formation and decay of radical pairs and fr
ee radicals. The data reflect the spatial distribution of reactants an
d products. Linewidths and relaxation data give information on the loc
ation and motional freedom of free radicals.